Someone looking 'pretty enough' that people are stunned is entirely subjective. Same with scary or any other physical attribute.

If this physical attribute effects groups of people, and not just one singularly mentally weak individual, this effect would not be just some weird freak occurrence. It is a tangible effect of viewing this person. Plus, I could argue that its not even purely physical attributes being viewed by individuals. The magic and science used to create space marine gene seed from primarchs, and primarchs from the emperor can easily be used to explain this effect of dread seen in individuals.

Space Marines kinda proves my point- for any given instance of you finding someone "stunned" by a marine I would wager I can find an instance of someone being quite willing to fight them.

Everything is canon though. And these Horus Heresy books that include this effect are some of the newer books in the series.

If the GEoM has the power to make people who see him weep at his perfection, obviously caused by his psychic nature, this can be used in matches. This effect has been seen to even blind primarchs before. I'm just saying you would need to have a character that has shown to have an extreme level of mental fortitude against magic powers to resist such effects. That your average human walking up to punch the GEoM would probably become catatonic at the sight of him.